This invention relates to homopolar dynamoelectric machines and more particularly to drum type homopolar machines having radially movable current collecting brushes.
Homopolar generators have been successfully designed for providing short duration pulses having peak current levels in excess of a million amperes DC. Such generators generally include a cylindrical rotor of either a drum or disc configuration, mounted in a stator and rotated about a central axis. A field coil encircling the rotor and connected to an external current supply provides an applied field excitation passing through the rotor. The applied magnetic field excitation is usually confined and directed by a ferromagnetic stator structure surrounding the field coil and all, or a portion of, the rotor. When the rotor is spinning, free electrons within the rotor experience an electromotive force resulting from their interaction with the applied magnetic field excitation. Brushes, positioned adjacent to the surface of the rotor, are then lowered onto the spinning rotor to allow an external current to flow under the influence of such electromotive force through return conductors to an external circuit and then back onto the rotor through additional brushes at a different location. Interaction between the discharge current and the applied magnetic field excitation creates a force which decelerates the rotor. It has been found that extremely high current pulses may be obtained by using a relatively low power conventional prime mover or a conventional low voltage, low amperage power source to store initial energy in the rotor by gradually motoring the rotor up to the desired rotational speed.
Drum type homopolar dynamoelectric machines include a stationary excitation system and a rotating drum composed of a combination of ferromagnetic and highly conductive materials configured such that a direct current output voltage is produced along the axial length of the drum. These machines incorporate a set of current collection members at either axial end of the rotor, which carry full load current. Homopolar dynamoelectric machines may operate as either a motor or generator and are particularly suited to transfer energy in short, high current pulses to a storage inductor and a final load consisting of, for example, a resistive-inductive system. The rotor of a drum type homopolar machine may include a cylindrical shell of a highly conductive, non-ferromagnetic material which supports the full load current. This member is bonded or shrunk onto a ferromagnetic inner cylindrical core which serves as the main rotor body and is directly attached to a drive or input shaft. Both components of the rotor are, preferably, homogeneous materials without segmentation or any combination or axial or circumferential grooves. The machine's internal electromotive force is generally confined to an axial zone along the center of the rotor between two outer current collection zones.
Since homopolar generators are used to provide high current, short duration pulses, which may be in excess of one million amperes, steps must be taken to minimize brush wear and to minimize heat generated in the current collection zones of the rotor. This is usually accomplished by providing brush actuators which are used to move the brush assemblies into and out of sliding electrical contact with the current collection zones of the rotor. In general, these brush actuators are located within the stator structure thereby requiring a stator structure which is large enough to contain the actuators. However, for certain homopolar generator applications, for example, airborne and space applications, it is desirable to reduce over-all weight and size of the machines.